1. Field of the Invention
The invention relates to joining optical fibres and particularly, but not exclusively to joining optical fibres at the customer end of telecommunications networks, often referred to as the secondary, or access network.
2. Description of the Related Art
Typically in telecommunications, joints are provided where it is desired to distribute optical fibres to customers. At the joint, optical fibres leading from a main, or primary network may be connected to fibres on a secondary network leading to individual customer locations. Hereinafter fibres from the main network will be described as being supplied along a primary cable as primary fibres, and fibres leading to customer locations will be described as being supplied in secondary cables as secondary fibres. However, such joints can be provided to connect optical fibres from two primary cables thereby increasing the total length of cable and it is to be understood that references to primary, or first, and secondary, or second, fibres and networks herein are made for ease of description and should not be taken as limiting.
In an installation for connecting optical fibres, an optical fibre cable is typically connected to a joint arrangement where the optical fibres (e.g. primary and secondary optical fibres) are joined. Such joints are usually provided underground and may be accessed via a man-hole to enable connection work and fibre routing to be carried out by an engineer, or installer.
Secondary optical fibres may be provided in enhanced performance fibre units (EPFU) that may comprise one or more fibres extending along a tube. The tube may be installed without optical fibres and when required, the optical fibres are blown through the tube using known techniques. Typically, the tubes are made from polyethylene and have a carbon loaded radially inner surface to increase conductivity, as disclosed, for example, in U.S. Pat. No. 4,952,021. This helps dissipate static charges developed during blowing that can hinder installation of fibres along the tube.
Primary cables typically comprise a plurality of optical fibre bundles, or units, each bundle typically comprising eight or twelve fibres. At a joint location, the optical fibres are separated and the individual optical fibres are managed and routed to housings, or splice trays, where they may be connected to secondary optical fibres leading to individual customer locations. An example of these housings, or splice trays, is disclosed in the applicant's earlier application, EP-A-0981775. Each housing comprises a first port into which fibres may be routed from a primary cable and a second port into which fibres may be routed from customer locations. Alternatively, the housings can be used to connect fibres from two primary cables.
FIG. 1 is a schematic illustration of an optical fibre installation comprising a joint arrangement 10 in which a plurality of housings 12, as disclosed in EP-A-0981775, are utilised for joining primary and secondary fibres. The arrangement comprises a plurality of housings, or splice trays, 12 each of which has respective inlet ports, not shown, for receiving primary and secondary optical fibres. The splice trays 12 are supported by an upright member, or joint spine 14, which extends from a base member 18 that typically takes the form of a generally flat plate. The base member 18 includes a plurality of spigots 20 each of which has an axially extending through-hole 22. The end of the through-hole 22 disposed on the outside of the joint, that is the end disposed remote from the surface of the base member from which the joint spine extends, is closed by a knockout 24.
In use, when it is desired to route fibres from a secondary cable 26 into the joint, the knockout 24 is removed and the tube 28 containing the fibres 30 of the secondary cable is inserted into the joint via the through-hole 22 as indicated by arrow 32. A heat shrink sleeve 34, previously threaded onto the tube 28, is then collapsed onto the tube and spigot 22 to seal the external entry point of the secondary cable 26 to the base member to prevent the ingress of moisture and dirt.
Collapse of the heat shrink sleeve 34 is caused by applying heat to it, typically by way of a flame. As observed by the Applicant, this limits the number of secondary cables 26 that can be inserted into a base member 18 of a given area, since it is necessary to leave sufficient space between each spigot 20 to ensure that when heat is applied to a heat shrink sleeve 34, it will not damage adjacent secondary cables.
Installing joint arrangements 10 is expensive and it is desirable that each installation should be kept relatively small and supply the maximum number of customers. On the primary side, primary cables comprising large numbers of primary fibres are available. For example, primary cables comprising twelve fibre bundles, each bundle containing eight optical fibres, (making ninety-six optical fibres in total) are known. Until recently, two fibres were needed to allow communication at a customer location; a first optical fibre transmitting light and a second optical fibre receiving light. Therefore, the exemplary primary cable having ninety-six primary fibres could be connected to a joint arrangement for connection to forty-eight customer locations
Recently, single fibre circuits have been introduced in which a single fibre both transmits and receives light. In this case, the exemplary primary cable having ninety-six primary fibres may be connected to a joint arrangement for connection to ninety-six customer locations, although often where a customer is a business, it will require more than one connection, and in some cases, the installation may include single and twin fibre circuits.